GNIRS Sensitivity Estimates

If you want to consult the monitoring of GNIRS sensitivity and throughput since 2011, please go here.

The following table provides estimated broadband magnitudes reached at 5 sigma (per spectral pixel) in 1 hour of integration, both without and with adaptive optics (AO). (This page gives information on sensitivity in imaging mode.) The values in the XJHK bands are at wavelengths that do not coincide with OH sky lines; at the wavelengths of OH lines the S/N will be lower, in many cases substantially so. The XJHK values assume twelve exposures of 300 seconds (5 minutes) each and ABBA nodding along the slit. (Longer exposures are possible and may result in improved sensitivity, but are not recommended at present, because the radioactive coatings in the current camera lenses create patches of bad pixels at a high rate. For the L and M bands it is assumed that the exposures are long enough that the noise is background-limited. The observing conditions assumed are 70%-ile image quality, 50%-ile cloud cover (clear), 80%-ile sky background (needed for X and J band observations only), 50%-ile water vapor and an airmass of 1.5.

The tabulated values are for a slit width (and thus resolution element) of 2 pixels and are for a single pixel on the spectral axis (thus, e.g., for a 2-pixel wide slit the sensitivity per resolution element is improved by sqrt(2) over the tabulated values). The values in the table and in the ITC also assume optimal summing of pixel values along the slit, with the number of pixels summed determined by the pixel scale and the image quality. Note that this is not optimal (i.e., weighted) extraction. The 5 sigma - 1 hour magnitudes are given at specific wavelengths near the center of each band which correspond to high atmospheric transmission and do not coincide with strong sky emission or absorption lines.

The values given are all for the long slit mode, i.e., with no prisms present. For observations in cross-dispersed mode, sensitivities are typically 0.1 magnitudes worse at X, J, and H, and 0.2 mag worse at K; there are modest reflection losses in the prisms as well as absorption losses at the red end of the K window.

AO correction is a strong function of wavelength. In the X and J bands the use of AO with incurs large slit losses. In the K band thermal emission from the warm AO system (which is roughly 90 percent transmissive) contributes significant thermal background, comparable to that of the telescope and the sky. Except in the K band at the highest resolving powers AO is not advised unless the science requires high angular resolution; higher sensitivity is achieved without AO. In the L band both the light losses and thermal emission are much higher, resulting in a much greater reduction in sensitivity than at shorter wavelengths. Indeed the high background prohibits use of AO in the latter band).

Important caveats:

In the X, J, H, and K bands, S/N values for targets much brighter than the magnitude values in the table cannot be linearly scaled from the values in the table, because for much brighter sources, shot noise dominates, rather than read noise or background fluctuations, which determine the noise when the target is faint. E.g., the S/N on an H=8 mag star will not be 10,000 times higher in one hour than the S/N on an 18 mag star observed for the same amount of time.

Values of S/N derived from this table that exceed several hundred are usually not realistic, not only because of the above shot-noise issues, but also because achieving very high S/N depends on many factors in addition to signal, background, and detector read noise. See here for further information.

Within each IR band the sensitivity is a very strong, rapid, and non-monotonic function of wavelength, because of wavelength dependent atmospheric transmission and sky emission. Applicants should use the ITC to estimate the signal-to-noise ratio at the precise wavelengths of interest and/or over the entire band of interest.

The AO sensitivities are derived from the expected performance rather than on-sky values. We believe that they are conservative. They may change during the next several months as we obtain more information about the performance of the instrument with AO.